The present invention concerns a frame construction for a press section of a paper machine in which there are at least two, preferably three successive and separate press nips in a direction of running of the web from which water is removed, the press nips substantially dewatering the web and being formed between two opposite press elements such as press rolls. The web runs through these nips supported by a dewatering and/or transfer fabric.
In prior art press sections of a paper machine such as the Valmet so-called Sym-Press (TM) press section, there have been horizontal beams above the press rolls both at the service side and at the operating side of the paper machine, which connect the front frame and the rear frame of the press section permanently together. In this connection and also in the following description, the front frame means the frame part that is situated in the running direction of the web at the front side, with a pick-up suction roll of the press being fixed, e.g., to the front frame. In a corresponding manner, the rear frame means the frame part situated at the rear side of the press section, i.e. closer to the drying section.
From the Valmet Finnish Patent Applications Nos. 823187, 842114, and 842115, paper machine press sections provided with separate press nips are previously known in which there are two or three succeeding, separate press nips in a running direction of the web substantially dewatering the web, these nips being formed between two press rolls. The web runs through the nips between two fabrics. At least in the first one of the nips, the dewatering takes place through both faces of the web as the nips are formed between two hollow-faced press rolls or between a hollow-faced roll and a smooth-faced roll. In the press section, the first upper or lower fabric acts as a pick-up fabric, onto which the web is transferred from the forming wire.
According to the Valmet Finnish Patent Application No. 823187, it is considered an important feature in the press section described therein, that the press section includes two upper fabrics and two lower fabrics. The first fabrics in the running direction of the web are press fabrics that receive water, and are arranged in a manner such that one of these first fabrics acts as a press fabric in the first nip, and the other fabric acts as a press fabric both in the first nip and in the second nip.
According to FIG. 3 of Finnish Application No. 823187, the press section is provided with a front frame, a rear frame, and with an intermediate frame between the same, these frame parts being interconnected by means of horizontal beams situated both at the service side and at the operating side of the paper machine. These beams interconnect the vertical frame parts above the press rolls. Under these circumstances, the concerned frame part is permanently fully closed from above, which makes the replacement of the press rolls and of the various fabrics more difficult and slow.
In connection with the prior art frame parts of these press sections, difficulties and relatively long standstills have occurred in relation to the replacement both of the press fabrics and of the press rolls. These problems have been increased with an increase in the widths of the paper machine, in particular because the press rolls have become much longer and heavier. These problems have also been increased by the fact that press fabrics which are made of plastic material and which are rigid in the transverse direction thereof, has started being used ever increasingly. These press fabrics cannot be jammed into a bundle because of their rigidity.